中文摘要：

采用正硅酸四乙酯（TEOS）的乙醇-水溶液为前驱体,氢氟酸为催化剂,结合溶胶-凝胶过程与CO2超临界流体干燥工艺,一步反应获得了密度为40-175mg·cm-3的单元气凝胶.以上述工艺为基础,通过逐层凝胶法、溶胶共凝法和梯度溶胶共凝胶法分别制备了三种密度梯度气凝胶样品,并研究了其功能梯度特性.结果表明：不同密度的气凝胶均具有粒径约为40-90nm球形颗粒构成的三维骨架结构,密度越低,骨架越疏松,峰值孔径越大,孔径分布也更为分散;三种方法制备的样品均具有明显的密度梯度,梯度特性由不连续到连续.动态热机械性能测试表明,随着密度的降低,气凝胶在低温（-100℃）和常温（25℃）下杨氏模量均有减小的趋势,其范围分别约为4.6×105-1.9×105Pa和5.0×105-2.1×105Pa.热学测试表明,随着密度的降低,气凝胶的热扩散系数增高,单位体积热容降低,而热导率则不成单调变化.

英文摘要：

Aerogels with densities in the range 40-175 mg·cm-3 were prepared using a tetraethyl orthosilicate（TEOS）ethanol-water solution as the precursor and hydrofluoric acid as the catalyst via a sol-gel process and CO 2 supercritical-fluid drying.The density-gradient aerogels were prepared using layer-by-layer gelation,sol co-gelation,and gradient-sol co-gelation methods and their gradient properties were studied systematically.The results show that aerogels with different densities all have a three-dimensional skeleton consisting of spherical particles of diameter about 40-90 nm.The lower the density is,the looser the skeleton and pore-size distributions are,and the larger the peak value of the pore size is.Gradient aerogels prepared via different methods exhibited graded,approximately gradient,or gradient distributions.Dynamic mechanical analysis indicates that the Young？s moduli of the aerogels at-100 and 25℃（changed from 4.6×10 5 to 1.9×10 5 Pa and from 5.0×10 5 to 2.1×10 5 Pa,respectively）tend to decrease with decreasing density.Thermal constants analysis shows that as the densities of the aerogels decrease,the thermal diffusion coefficients increase and the specific heat capacities decrease,but the thermal conductivities do not change monotonically.